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Zebra Stripes in Outer Space

In an article recently accepted in the Journal of Geophysical Research, Dr. Solène Lejosne, post-doctoral fellow at the UC Berkeley’s Space Sciences Laboratory and Pr. Juan G. Roederer, Professor Emeritus from the Geophysical Institute of Alaska, propose a new theory to explain a feature of the Earth’s Van Allen radiation belt particles that has defied satisfactory explanation for half a century.

This feature has been given the name “zebra stripes” by space physicists because it shows up as a zebra-like pattern in energy spectrograms, indicating highly organized compressions and expansions of the radiation belt particles as they circle the Earth. Lejosne and Roederer liken their proposed mechanism to what happens in multi-lane highway traffic during rush hour, when an initial group of evenly spaced cars is segregated into high/low density bunches according to their acceleration power and the varying speed limits along their route.

In the case of the Van Allen belt particles, it is the upper atmosphere that acts as the “traffic regulator”: ionospheric zonal winds in equatorial regions, which during quiet solar activity conditions blow eastward around midnight and westward around noon, induce electric fields which speed up or slow down the drift of the particles around the planet.

It should be noted that this is the first time that a direct physical link between zonal ionospheric winds and the near-earth region of the radiation belt has been proposed. Earlier theories for zebra stripes assumed the action of complicated mechanisms of wave resonances, diffusion, or inductive acceleration, but were unable to explain all observed features of this phenomenon.

Since zebra stripes represent the modulation of radiation belt particle intensity, they are also a signature of modulation of the radiation dose to which spacecraft and their electronics are exposed. Understanding the origin and behavior of this phenomenon is thus also of interest to satellite communications technology and other space ventures.

FYI: The radiation belts were discovered by Van Allen in 1957; they consist of energetic electrons, protons and positive ions trapped in the planetary magnetic field, circling rapidly around the field lines, bouncing north and south along them, and drifting around the planet. All magnetized planets have radiation belts; in the terrestrial case, most of these particles originate in the magnetospheric tail from where they are propelled earthwards during solar wind disturbances, being accelerated to high energies on the way. A fraction of these particles don’t make it into the trapping regions closer to Earth: they precipitate into the polar atmosphere and cause the auroras.

The appearance of structured peaks and valleys called zebra stripes in the spectrograms of energetic electrons and ions trapped below L~3 evidences the effect of high altitude ionospheric winds on radiation belt intensity.